Progressive display devices display all lines of an image every refresh. In contrast, interlaced display devices, such as NTSC and PAL television displays, typically display images using even and odd line interlacing. To display interlaced video on a progressive display, video rendering systems have to generate pixel data for scan lines that are not received in time for the next frame update. This process is called de-interlacing. For applications such as High Definition Television (HDTV), it is often necessary to display video in a different resolution or in a window on another video image.
A problem in processing video images exists particularly for personal computers having multimedia capabilities since interlaced video information received from conventional video tapes, cable television broadcasters (CATV), digital video disks (DVD's) and direct broadcast satellite (DBS) systems must be de-interlaced for suitable display on a progressive (non-interlaced based) display device, and scaled and re-interlaced by a television encoder for display on an interlaced display device.
A current video compression standard, known as MPEG-2 specifies the compression format and decoding format for interlaced and non-interlaced video picture information. MPEG-2 video streams have picture data divided as blocks of data. These blocks of data are referred to as macroblocks in the MPEG-2 standard. Generally, a macroblock of data is a collection of Y, Cr, Cb (color space) blocks which have common motion parameters. Therefore, a macroblock of data contains a section of the luminance component and spatially corresponding chrominance components. A macroblock of data can either refer to source, decoded data or to the corresponding coded data elements. Typically, a macroblock of data (macroblocks) consists of blocks of 16 pixels by 16 pixels of Y data and 8 by 8, or 16 by 16 pixels of Cr and Cb data in one field or frame of picture data.
Generally, in MPEG-2 systems, two fields of a frame may be coded separately to form two field pictures. Alternatively, the two fields can be coded together as a frame. This is known generally as a frame picture. Both frame pictures and field pictures may be used in a single video sequence. A picture consists of a luminance matrix Y, and two chrominance matrices (Cb and Cr).
MPEG-2 video streams also include data known motion vector data that is solely used by a decoder to efficiently decompress the encoded macroblock of data. A motion vector, referred to herein as a decoding motion vector, is a two-dimensional vector used for motion compensation that provides an offset from a coordinate position in a current picture to the coordinates in a reference picture. The decoder uses the decoding motion vector data stream to reference pixel data from frames already decoded so that more compact difference data can be sent instead of absolute data for those referenced pixels or macroblocks. In other words, the motion vector data is used to decompress the picture data in the video stream. Also, zero decoding motion vectors may indicate that there was no change is pixel data from a previously decoded picture.
Several basic ways of de-interlacing interlaced video information include a “weave” method and a “bob” method. With the “weave”, or merge method, successive even and odd fields are merged. Each frame to be displayed is constructed by interleaving the scan lines of a pair of fields. This “weave” method is generally most effective with areas of a picture that do not have motion over successive frames because it provides more pixel data detail for non-moving objects. However, when motion does occur, artifacts appear in the form of double images of a moving object. An artifact called “Comb Tearing” or “Feathering” appears around the periphery of a horizontally moving object causing poor image quality. Images with vertically motion also have artifacts.
In contrast to the “weave” method, the “bob” method displays single fields as frames. The missing scan lines are interpolated from available lines in the field making the frame rate the same as the original field rate. The most often used methods are line repetition, line averaging and edge-adaptive spatial interpolation. Again, this de-interlacing method is also not typically used with some form of motion detection so that non-moving images can appear to be blurry from loss of image detail. This can result from inaccurate interpolation of pixel data. The “bob” technique introduces flicker that is noticeable in video sequences with no motion. This occurs because even when the scene is static, two different frames are created—one based on the even field and one based on the odd field. These frames are generally different. Where they are different, flicker occurs as the display alternates between the two frames.
There are a number of techniques categorized as motion adaptive de-interlacing. These use different de-interlacing strategies in picture areas with and without motion. Generally, “bob” is used in picture areas with motion and “weave” is used in picture areas without motion. Often, separate de-interlacers and/or separate motion detection hardware is used to carryout the above methods, as well as separate hardware for scaling and re-interlacing video (TV encoder). However, separate de-interlacers, motion detection, scaling and re-interlacing hardware can add additional cost to a graphics processor.
Graphics processors are known to include 2D/3D engines that fetch data from a frame buffer and blend pixels together to render an image and place the blended data back in the frame buffer. The frame buffer is memory accessible by the graphics processor. Such graphics processors are also known to include display engines which obtain rendered images from the frame buffer and may subsequently perform simple deinterlacing operations (such as “bob” and “weave”), but do not typically rewrite the deinterlaced information back to the frame buffer. As known in the art, the specifics of operations supported by 2D/3D engines vary. Also, it is not uncommon among 2D/3D engines for the same operation to use a different number of passes on different chips. Lighting and multi-texture affects are examples of features where different implementations partition the signal processing steps differently to achieve a tradeoff between die area, complexity, memory bandwidth, and performance. The feature sets of 2D/3D engines evolve rapidly to make them more and more efficient at the tasks for which they are most frequently programmed.
Accordingly, there is a need for an improved graphics processor that performs de-interlacing, weaving and scaling image rendering for television encoding.